1. Field of the Invention
This invention relates generally to the cell configuration and fabrication process of Power MOSFET devices. More particularly, this invention relates to a novel and improved trench MOSFET cell structure, and improved process of fabricating a trench MOSFET with shallow trench structures.
2. The Prior Arts
In order to resolve restrict of a high gate charge introduced in a trench MOSFET of conventional configurations, a shallow trench structure was disclosed by decreasing a trench depth, please refer to FIG. 1A for an N-channel trench MOSFET of prior art. However, when forming the shallow trench structure during fabrication process, a trenched gate contact 109 into a shallow trenched gate may penetrate through a gate oxide and short to a drain because the Cdpoly (trenched gate contact depth, as shown in FIG. 1A) is about 1.5 times deeper than Cdsi (trenched source-body contact depth into an epitaxial layer, as shown in FIG. 1A) as the result of faster etching rate in a doped poly silicon than in a silicon layer.
Another disadvantage of the prior art is that, as illustrated in FIG. 2 (the upper curve), to prevent the increase of Rds (Resistance between Drain and Source), a difference between Td (Trench depth, FIG. 1A) and Pd (P body depth, FIG. 1A) must be kept larger than 0.4 μm, thus forming a large overlap region between the trenched gate and the epitaxial layer, therefore greatly increasing Qgd (gate to drain charge, FIG. 1A).
To further reduce Qgd, a trench MOSFET with a thick bottom oxide of prior art (U.S. Pat. No. 5,126,807) was disclosed, as shown in FIG. 1B. However, according to the prior art, the thick bottom oxide 129 is formed by LOCOS (Local Oxidation of Silicon underneath a trench bottom) having a bird's beak region 131 with a nitride layer 130 on sidewalls of the trenches, resulting in a deterioration of a breakdown voltage as result of a weak corner oxide formed in the trench bottom.
Accordingly, it would be desirable to provide a novel trench MOSFET with shallow trench structure and improved fabrication process to maintain lower Rds, lower Qgd and higher breakdown voltage.